Electronic filter



NOV. 2, 1954 EDWARDS 2,693,565

ELECTRONIC FILTER Original Filed June 8, 1949 Fly. 2

I90 c i a 34 1 qb-{i-j 290 3/0 INVENTOR. JO/VA THAN EDWARDS A a/m5 X5 United States Paten ELECTRONIC FILTER Jonathan Edwards, San Diego, Calif.

Original application June 8, 1949, Serial No. 97,869, now

Patent No. 2,629,014, dated February 17, 1953. Divided and this application June 11, 1952, Serial No. 298,485

1 Claim. (Cl. 321-) (Grautedunder Title 35, U. S. Code'(1952), sec. 266) This invention relates to filters for removing the ripple from a uni-directional voltage power supply, such as is used in'electronic circuit work.

I This application is a division of application Serial No. 97,869, .filed' June 8, 1949, now Patent No. 2,629,014, granted February 17, 19.53.

The use of a non-linear impedance, such as a saturated diode, biased to an operating point just above the knee of its characteristic curve, and inserted between a ripplecontaining power supply and a load, is known in the art. The operating point is determined by the magnitude of the current flowing through the saturated diode, and hence through the load. Therefore, for each different load demand, it is necessary to have a different diode, inasmuch as each diode has a single characteristic curve.

It is an object of this invention to provide an improved non-linear ripple filter circuit which may be readily ad justed for different load currents drawn from the source of uni-directional voltage without necessity for changing the filter element.

It is another object of this invention to provide a filter circuit for suppressing the ripple in a source of uni-directional voltage which embodies not only the advantages of a non-linear impedance, but also has means for directly suppressing'the' ripple by feeding it backin phase opposition, i'. e; negative feedback.

Another object of this invention is to provide a simplified inductive reactance in the form of a tube which may be substituted for the conventional choke coil, with a saving in space and weight and in power lost in the filtering element.

A further object of this invention is to accomplish the above outlined objects and still insert the filter element in the positive lead of the voltage source, rather than in the negative or ground lead, thereby simplifying use of the voltage source for many applications.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following description.

A principal feature of the instant invention resides in the employment of a screen grid tube, which has a family of characteristic curves, each one representing a typical non-linear impedance saturation curve, the particular curve in the family being determined by the voltage bias on the control grid of the tube. Thus, for any load current desired the control grid bias on the screen grid tube may be so chosen that the operating point lies just above the knee of the curve, thus giving optimum operating conditions, i. e. a high A. C. resistance, and a relatively low D. C. resistance.

Use of a screen grid tube in the above-described manner readily lends itself to the operation of negative ripple current feedback to the tube also, so that in addition to utilizing the saturation characteristics of the screen grid tube, the tube may serve as a means by which degenerative ripple feedback can be introduced into the system to further suppress the ripple from the uni-directional voltage power.

Use of a screen grid tube in the above outlined manner also readily lends itself to the application of a quadrature voltage through the control grid of the tube, whereby it may be made to appear as an inductive reactance-the full equivalent of a bulky and weighty choke coil, and with the added advantage of the saturation characteristics above discussed.

2,693,565 Patented Nov. 2, 1954 In accordance with the above general description, several embodiments of the instant invention will now be described with reference to the accompanying drawing, wherein:

Fig. 1 illustrates a-simple form *of the instant invention;

ig. 2-sh'ows a modification in which degenerative ripple feedback isapplied to the filter tube;

Fig. 3 shows the use of the screen grid filter tube with quadrature feedback, giving it the added advantage of appearing as an inductive reactance in the power supply circuit; and

Figs. 4, 5, and 6 illustrate how the screen grid filter tube may be placed in the positive lead of the power supply rather than in the negative lead.

Referring to Fig. 1, .11 designates the usual source of ripple-containing uni-directional voltage, consisting of a transformer 12, the secondary winding 13 of which is connected to the anodes 14 and 16, respectfully, of a double rectifier 17. Thecathode 18 of the rectifier 17 'is connected to the positive output terminal 19, while the midpoint 21 of the secondary winding 13 is connected, through a filter circuit to be described, to the negative output terminal 22. Across the terminals 19 and 22 is connected any desired load, represented by the impedance 23.

In accordance with the instant invention, a screen grid tube 24 has its'anode-cathode connected in series with the negative lead of the power supply. In Fig. 1, the anode 26 of the tube 24 is shown connected directly to the negative terminal 22, while the cathode 27 is connected through an adjustable impedance 28, to the point 21 constituting the negative terminal of the basic-supply source 11. The control grid 29 ofthe tube 24 is connected to one terminal-of the resistor 28, coinciding electrically with the point '21,. so that the IZ voltage drop in the cathode impedance 28 provides a negative bias from the cathode 27 to the grid 29. The screen grid '31 of the tube 24 is returned through a resistor 32 to the positive terminal 19 of the power supply. Numerals 33 represent conventional filter capacitors.

It is characteristic of screen grid tubes suchas'24 that when plate current is plotted as ordinate against plate voltage as abscissa, a series of typical saturation curves are derived rising steeply and then bending sharply at a knee. The location of each member of the curve is determined by the bias on the control grid 29. It is thus seen that in the instant invention as illustrated in Fig. 1, for any given load 23 drawing a particular current, it is only necessary to adjust the impedance 28 so that there is chosen that tube characteristic which has its knee just below the operating point as determined by the load current through 23. In this way, the screen grid tube 24 may be made to serve as an efiicacious ripple filter for different loads 23, it being merely necessary to adjust the impedance 28 whenever the load 23 is changed. As is known from prior usage of saturable diodes, by placing the operating point just above the knee of the curve, a relatively high A. C., or ripple, resistance is obtained with only a relatively low D. C. resistance.

Fig. 2 illustrates the employment of the instant invention, with the addition of negative, or degenerative, ripple feedback, so that in addition to furnishing the saturation characteristic above described, the screen grid tube 24a also deliberately and specifically suppresses the ripple voltage, by means of negative feedback.

As shown in Fig. 2, the screen grid tube 24a is inserted in the negative lead between the point 21a and the negative terminal 22a. This figure difiers principally from Fig. l in that it employs a capacitor 34, which feeds back degenerative or negative ripple signals to the grid 29a, and the capacitor 36, the latter feeding back degenerative ripple signals to the screen grid 31a. Both feedbacks tend to further suppress the passage or ripple voltage power through the tube 24a.

Fig. 2 also illustrates an alternative manner of deriving the adjustable bias for the grid 29a. In this case, the cathode impedance is made in the form of a potentiometer 28a, the slider of which is connected through a resistor 37 to the control grid 29a.

In Fig. 3, the basic circuit shown in Fig. 1 has been modified by the use of a reactive circuit to apply a quadrature voltage to the grid 2% of the tube 24b, so that the tube presents the appearance of an inductive reactance between the terminals 21b and 22b.

This is done by connecting a circuit consisting of capacitor 38 in series with resistor 39 between the terminal 21b and the anode 26b. 40 is a small blocking capacitor. The midpoint between the capacitor 38 and resistor 39 is connected to the control grid 29b as shown. When the resistance of the resistor 39 is made much larger than the reactance represented by the capacitor 38, it may be readily shown that the A. C., or ripple, voltage applied to the control grid 2% lags the plate-tocathode voltage of the tube 24b by substantially 90, thereby causing the tube 24b to appear substantially as an inductive reactance to the A. C. ripple voltage appearing in the power supply circuit. In this way the advantages of a choke coil are achieved without the bulk and weight of the latter, and without the undue power loss resulting from the use of a choke.

In the modifications illustrated in Figs. l-3, it has been necessary to place the tube 24 in the negative lead of the power supply in order to obtain a ready source of high enough biasing voltage for the screen grid 31. In the embodiment shown in Fig. 4, placement of the screen grid filter tube 240 in the positive lead of the power supply has been achieved by making use of an auxiliary single rectifier 41, having its anode 42 connected to the anode 140 of the principal double rectifier 17c, and having its cathode 43 connected to the screen grid 31c of the tube 240. A capacitor 44, also connected to the screen grid 31c, serves to build up a charge of current rectified in the rectifier 41, so as to maintain the bias on the screen grid 310 at substantially the full output voltage of the rectifier-transformer combination 17c-12c.

As in Fig. 2, degenerative A. C. ripple voltage may be applied back to the control grid 290 through a capacitor 340; and the capacitor 44, in addition to holding the charge bias for the screen grid 310, also serves to feedback degenerative A. C. ripple voltage. Proper bias for the control grid 29c is achieved in Fig. 4, in a manner similar to that shown in Fig. 2, by the use of a potentiometer 28c, the slider of which is connected to the control grid 29c.

Fig. is substantially the same as is Fig. 4, with the addition of a series-connected RC circuit 46, 47, inserted 4 in the circuit at the rectifier anode 42a for the purpose of cutting down the bias voltage on the screen grid 31d.-

In Fig. 6, the use of any suitable amplifier 48 is shown for amplifying the ripple voltage appearing across the load 23:: before applying it in degenerative fashion to the control grid 29e of the filter tube 24c.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claim the invention may be practiced otherwise than as specifically described.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

A ripple suppressed source of unidirectional voltage comprising, a center-tapped source of alternating voltage, the center tap thereof constituting the negative terminal for the source of unidirectional voltage, a double rectifier diode having a pair of anodes, each anode being connected, respectively, to the terminals of said alternating voltage source, a screen grid tube having its anode connected to the cathode of said diode, the cathode of said screen grid tube being connected to the positive terminal of the source of unidirectional voltage, a single rectifier diode connected between one of the terminals of said alternating voltage source and the screen grid of said tube, a capacitor connected between said screen grid and the negative terminal of said source of unidirectionalvoltage, and impedance connected to and in series with the cathode of said screen grid tube, circuit means connecting said control grid to a point of biasing potential with respect to the cathode of said screen grid tube, and means for adjusting the biasing potential on said control grid.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,106,793 Burton Feb. 1, 1938 2,309,852 Lewis Feb. 2, 1943 2,456,638 Kenyon Dec. 21, 1948 

